Final answer:
Electromagnetic waves, consisting of electric and magnetic fields, can generate electric fields that are detectable by EEG at the scalp. Electric fields are denoted by E and they are part of the self-propagating electromagnetic waves that travel at the speed of light.
Step-by-step explanation:
Electromagnetic waves create an electric field which may be measured at the scalp using surface electrodes. These waves consist of changing electric and magnetic fields that propagate through space. We use E to denote electric fields and B to denote magnetic fields. These mutually-regenerating fields can travel through empty space at a speed of approximately 3 × 108 m/s, and they can also travel through different mediums like water and air, although at slower speeds.
Specifically, when we consider technologies such as EEG (electroencephalography) that measure brain activity, it is the electric fields generated by the neurons' activity that are detected at the scalp. These fields are closely related to magnetic fields, as they propagate together as an electromagnetic wave.